Touch sensing circuit and a signal demodulating method

ABSTRACT

A touch sensing circuit including a plurality of touch sensing channels is provided. Each of the touch sensing channels includes at least one operational amplifier circuit and a demodulating circuit. The operational amplifier circuit is configured to receive a touch sensing signal, and amplify the touch sensing signal. The operational amplifier circuit includes an operational amplifier. The operational amplifier has an inverting end, and the inverting end is coupled to a direct-current voltage. The demodulating circuit is coupled to the operational amplifier circuit. The demodulating circuit is configured to demodulate the amplified touch sensing signal by mixing the amplified touch sensing signal with a demodulating signal. In addition, a signal demodulating method is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of and claims thepriority benefit of a prior application Ser. No. 15/009,753, filed onJan. 28, 2016, now allowed. The entirety of the above-mentioned patentapplication is hereby incorporated by reference herein and made a partof this specification.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention generally relates to an electrical circuit and a signalprocessing method, in particular, to a touch sensing circuit and asignal demodulating method.

2. Description of Related Art

As the technology of touch panel develops, touch panels become widelyused as the screens of electronic devices, such as mobile phones, laptopcomputers, and tablet computers. Touch panels allow the user to input oroperate the electronic devices more conveniently and make the interfacemore user-friendly and convenient.

Generally speaking, the electrode structure of a capacitive touch panelincludes multiple scanning electrodes and multiple sensing electrodes.In terms of actual application, the scanning electrodes are used toreceive the driving signals inputted by a panel controller/driver, so asto drive the touch panel to sense the touch of the user. The sensingelectrodes are used to generate touch sensing signals corresponding tothe user's touch. In the conventional technology, AFE (analog front-end)hardware modules are used as interface sensors of many kinds to digitalsystems, providing hardware modularity. For touch sensing circuits, AFEcircuits are also configured to compensate the touch sensing signals byusing a compensation capacitor. However, the compensation capacitor mayoccupy large circuit area, so as to increase manufacturing costs. Inaddition, the touch sensing circuit demodulates the touch sensingsignals with a sine wave signal. Accordingly, a high-bit multiplier forsignal demodulation is necessary, thereby increasing the complexity ofthe touch sensing circuit, and reducing circuit performance.

Hence, how to design a touch sensing circuit that has simplified circuitstructure and satisfactory circuit performance without increasing themanufacturing costs is one of the most important topics in the pertinentfield.

SUMMARY OF THE INVENTION

Accordingly, the invention is directed to a touch sensing circuit and asignal demodulating method, capable of simplifying circuit structure andenhancing circuit performance.

The invention provides a touch sensing circuit including a plurality oftouch sensing channels. The plurality of touch sensing channels eachinclude at least one operational amplifier circuit and a demodulatingcircuit. The at least one operational amplifier circuit is configured toreceive a touch sensing signal and amplify the touch sensing signal. Theat least one operational amplifier circuit includes an operationalamplifier having an inverting end. The inverting end is configured toreceive the touch sensing signal and a direct-current voltage and tocompensate the received touch sensing signal by the direct-currentvoltage. The demodulating circuit is coupled to the at least oneoperational amplifier circuit. The demodulating circuit is configured todemodulate the amplified touch sensing signal by mixing the amplifiedtouch sensing signal with a demodulating signal.

In an exemplary embodiment of the invention, the at least oneoperational amplifier circuit includes a plurality of operationalamplifier circuits. Each of the touch sensing channels includes amultiplexer circuit. The multiplexer circuit is coupled between theoperational amplifier circuits and the demodulating circuit. Themultiplexer circuit is configured to sequentially select one of theoperational amplifier circuits, and pass the amplified touch sensingsignal to the demodulating circuit.

In an exemplary embodiment of the invention, the amplified touch sensingsignal is a periodical signal having a period. The demodulating circuitdemodulates the amplified touch sensing signal to extract a carriersignal from the amplified touch sensing signal during a part timeinterval of the period.

In an exemplary embodiment of the invention, the operational amplifierfurther has an output end. The at least one operational amplifiercircuit further includes an impedance device coupled between theinverting end and the output end.

In an exemplary embodiment of the invention, the demodulating circuitincludes a mixer circuit, a filter circuit and a gain amplifier circuit.The mixer circuit is coupled to at least one operational amplifiercircuit. The mixer circuit is configured to receive and mix theamplified touch sensing signal and the demodulating signal to generatethe demodulated touch sensing signal. The filter circuit is coupled tothe mixer circuit. The filter circuit is configured to receive thedemodulated touch sensing signal, and reduce noise of the demodulatedtouch sensing signal. The gain amplifier circuit is coupled to thefilter circuit. The gain amplifier circuit is configured to receive andamplify the demodulated touch sensing signal that the noise is reduced.

In an exemplary embodiment of the invention, the demodulating circuitfurther includes an analog-to-digital converter circuit. Theanalog-to-digital converter circuit is coupled to the mixer circuit andlocated before the mixer circuit or after the mixer circuit. Theanalog-to-digital converter circuit is configured to convert the touchsensing signal of an analog format to the touch sensing signal of adigital format.

The invention provides a touch sensing circuit including an operationalamplifier circuit and a demodulating circuit. The operational amplifiercircuit is configured to receive a touch sensing signal, and amplify thetouch sensing signal. The operational amplifier circuit comprises aninverting end. The inverting end is configured to receive the touchsensing signal and a direct-current voltage and to compensate thereceived touch sensing signal by the direct-current voltage. Thedemodulating circuit is coupled to the operational amplifier circuit.The demodulating circuit is configured to demodulate the amplified touchsensing signal by mixing the amplified touch sensing signal with ademodulating signal.

In an exemplary embodiment of the invention, the amplified touch sensingsignal is a periodical signal having a period. The demodulating circuitdemodulates the amplified touch sensing signal to extract a carriersignal from the amplified touch sensing signal during a part timeinterval of the period.

In an exemplary embodiment of the invention, the touch sensing circuitfurther including an impedance device. The impedance device is coupledbetween the inverting end and an output end of the operational amplifiercircuit.

In an exemplary embodiment of the invention, the demodulating circuitincludes a mixer circuit, a filter circuit and a gain amplifier circuit.The mixer circuit is coupled to the operational amplifier circuit. Themixer circuit is configured to receive and mix the amplified touchsensing signal and the demodulating signal to generate the demodulatedtouch sensing signal. The filter circuit is coupled to the mixercircuit. The filter circuit is configured to receive the demodulatedtouch sensing signal, and reduce noise of the demodulated touch sensingsignal. The gain amplifier circuit is coupled to the filter circuit. Thegain amplifier circuit is configured to receive and amplify thedemodulated touch sensing signal that the noise is reduced.

In an exemplary embodiment of the invention, the demodulating circuitfurther includes an analog-to-digital converter circuit. Theanalog-to-digital converter circuit is coupled to the mixer circuit andlocated before the mixer circuit or after the mixer circuit. Theanalog-to-digital converter circuit is configured to convert the touchsensing signal of an analog format to the touch sensing signal of adigital format.

The invention provides a signal demodulating method adapted to a touchsensing circuit. The signal demodulating method includes: configuring atleast one operational amplifier circuit, wherein the at least oneoperational amplifier circuit includes an operational amplifier havingan inverting end; receiving, at the inverting end, at least one touchsensing signal and a direct-current voltage to amplify the at least onetouch sensing signal by compensating the received at least one touchsensing signal by the direct-current voltage; and demodulating theamplified at least one touch sensing signal by mixing the amplified atleast one touch sensing signal with a demodulating signal.

In an exemplary embodiment of the invention, the at least one touchsensing signal comprises a plurality of touch sensing signals, and thesignal demodulating method further includes: sequentially selecting oneof the amplified touch sensing signals to be demodulated.

In an exemplary embodiment of the invention, the amplified at least onetouch sensing signal is a periodical signal having a period. In the stepof demodulating the amplified at least one touch sensing signal bymixing the amplified at least one touch sensing signal with thedemodulating signal, the amplified at least one touch sensing signal isdemodulated to extract a carrier signal from the amplified at least onetouch sensing signal during a part time interval of the period.

The invention provides a signal demodulating method adapted to a touchsensing circuit comprising an operational amplifier. The signaldemodulating method includes: receiving, at an inverting end of theoperational amplifier, a touch sensing signal and a direct-currentvoltage; amplifying the touch sensing signal by compensating the touchsensing signal by the direct-current voltage; and demodulating theamplified touch sensing signal by mixing the amplified touch sensingsignal with a demodulating signal.

According to the above descriptions, in the exemplary embodiments of theinvention, the operational amplifier circuit compensates the receivedtouch sensing signal via a direct-current voltage, and the demodulatingcircuit demodulates the amplified touch sensing signal with ademodulating signal, such that circuit structure is simplified andcircuit performance is enhanced.

In order to make the aforementioned and other features and advantages ofthe invention comprehensible, several exemplary embodiments accompaniedwith figures are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 and FIG. 2 respectively illustrate a capacitive touch sensingscheme and an equivalent circuit thereof according to an embodiment ofthe invention.

FIG. 3 illustrates a schematic block diagram of a touch sensing channelaccording to an embodiment of the invention.

FIG. 4 to FIG. 8 respectively illustrate schematic waveforms ofdemodulating signals according to a plurality of embodiments of theinvention.

FIG. 9 illustrates a schematic circuit diagram of a touch sensingchannel according to an embodiment of the invention.

FIG. 10 to FIG. 11 respectively illustrate schematic diagrams of signaldemodulation according to a plurality of embodiments of the invention.

FIG. 12 illustrates a schematic circuit diagram of a touch sensingchannel according to another embodiment of the invention.

FIG. 13 illustrates a schematic circuit diagram of a touch sensingchannel according to another embodiment of the invention.

FIG. 14 illustrates a schematic block diagram of a touch sensing channelaccording to another embodiment of the invention.

FIG. 15 is a flowchart illustrating steps of a signal demodulatingmethod in an embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

The term “coupling/coupled” used in this specification (includingclaims) of the disclosure may refer to any direct or indirect connectionmeans. For example, “a first device is coupled to a second device”should be interpreted as “the first device is directly connected to thesecond device” or “the first device is indirectly connected to thesecond device through other devices or connection means.” In addition,the term “signal” can refer to a current, a voltage, a charge, atemperature, data, electromagnetic wave or any one or multiple signals.

FIG. 1 and FIG. 2 respectively illustrate a capacitive touch sensingscheme and an equivalent circuit thereof according to an embodiment ofthe invention. Referring to FIG. 1 and FIG. 2, a capacitive touch panel100 of the present embodiment includes a plurality of touch electrodesarranged in an array. The touch electrodes are categorized into scanningelectrodes 110 and sensing electrodes 120. A touch controller/driver mayoutput driving signals 112 to the scanning electrodes 110, so as todrive the sensing electrodes 120 to sense a gesture 200. The gesture 200performs on the capacitive touch panel 100 and changes surfacecapacitances of the areas between the scanning electrodes 110 and thesensing electrodes 120 that the gesture 200 touches. The sensingelectrodes 120 senses capacitance variations thereof, and transmitstouch sensing signals 114 to a touch sensing circuit 300. Theinformation related to the gesture 200 are carried on the touch sensingsignals 114. In an embodiment of the invention, the touch sensingcircuit 300 may include a plurality of touch sensing channels to processthe touch sensing signals 114 transmitted from the sensing electrodes120 of different rows or columns.

FIG. 3 illustrates a schematic block diagram of a touch sensing channelaccording to an embodiment of the invention. Referring to FIG. 3, thetouch sensing channel 310 of the present embodiment may serve as ananalog front-end (AFE) circuit for processing the received touch sensingsignal 114, and further output the processed touch sensing signal 116 toa circuit of the next-stage. In an embodiment of the invention, thetouch sensing channel 310 may include a set of analog signalconditioning circuitry that uses operational amplifiers, filters, andsometimes application-specific integrated circuits for sensors and othercircuits to provide a configurable and flexible electronics functionalblock, needed to interface a variety of sensors to an analog-to-digitalconverter or in some cases to a microcontroller.

In the present embodiment, the touch sensing channel 310 includes anoperational amplifier circuit 312 and a demodulating circuit 314. Theoperational amplifier circuit 312 receives and amplifies the touchsensing signal 114, and outputs the amplified touch sensing signal Vs tothe demodulating circuit 314. The demodulating circuit 314 is coupled tothe operational amplifier circuit 312. The demodulating circuit 314demodulates the amplified touch sensing signal Vs by mixing theamplified touch sensing signal Vs with a demodulating signal Vde. In thepresent embodiment, a waveform of the demodulating signal Vde includes aflat region. In a preferable embodiment, the demodulating signal Vde maybe a square wave, but the invention is not limited thereto. Any signalthat a waveform includes a flat region may be adopted for thedemodulating signal.

FIG. 4 to FIG. 8 respectively illustrate schematic waveforms ofdemodulating signals according to a plurality of embodiments of theinvention. Referring to FIG. 4 to FIG. 8, each of the demodulatingsignals includes a flat region in the exemplary embodiments. Forexample, the demodulating signal Vde0 illustrated in FIG. 4 is a squarewave having a relatively wide flat region FR0. The demodulating signalVde1 illustrated in FIG. 5 includes a flat region FR1, and has two edgesthat slopes gradually change. The demodulating signal Vde2 illustratedin FIG. 6 includes a flat region FR2, and has two edges with constantslopes. The demodulating signal Vde3 illustrated in FIG. 7 includes aflat region FR3, and has a left edge with a constant slope and a rightedge that the slope gradually changes. The demodulating signal Vde4illustrated in FIG. 8 includes a flat region FR4, and has a left edgethat the slope gradually changes and a right edge with a constant slope.However, the foregoing waveforms are not intended to limit the inventionbut may be adjusted according to the design.

FIG. 9 illustrates a schematic circuit diagram of a touch sensingchannel according to an embodiment of the invention. Referring to FIG.9, the operational amplifier circuit 412 of the present embodimentincludes an operational amplifier 411 and an impedance device 413. Theoperational amplifier 411 has an inverting end, a non-inverting end, andan output end. The inverting end is coupled to a direct-current voltageVDC. The non-inverting end is coupled to a ground voltage. The impedancedevice 413 is coupled between the inverting end and the output end. Inthe present embodiment, the inverting end is coupled to thedirect-current voltage VDC for signal compensation, and thus acompensation capacitor is unnecessary. The circuit structure of theoperational amplifier circuit 412 is further simplified.

In the present embodiment, the operational amplifier circuit 412amplifies the touch sensing signal 114, and outputs the amplified touchsensing signal Vs to the demodulating circuit 414. In the presentembodiment, the demodulating circuit 414 includes a mixer circuit 415, afilter circuit 417 and a gain amplifier circuit 419. The mixer circuit415 is coupled to the operational amplifier circuit 412. The mixercircuit 415 receives and mixes the amplified touch sensing signal Vs andthe demodulating signal Vde to generate the demodulated touch sensingsignal Vs′. The filter circuit 417 is coupled to the mixer circuit 415.The filter circuit 417 receives the demodulated touch sensing signalVs′, and reduces noise of the demodulated touch sensing signal Vs′. Thegain amplifier circuit 419 is coupled to the filter circuit 417. Thegain amplifier circuit 419 receives and amplifies the demodulated touchsensing signal Vs' that the noise is reduced. The gain amplifier circuit419 outputs the processed touch sensing signal 116 to the circuit of thenext-stage.

In the present embodiment, the demodulating signal Vde is generated viaa signal generator circuit 500. The signal generator circuit 500 may beembedded inside the demodulating circuit 414 or arranged outside of thedemodulating circuit 414. The invention is not limited thereto. Inaddition, the demodulating signal Vde of the present embodiment may be asquare wave, and the waveform thereof includes a flat region. Therefore,the mixer circuit 415 for mixing the amplified touch sensing signal Vsand the demodulating signal Vde may include a low-bit multiplier forsignal demodulation.

In the present embodiment, the mixer circuit 415, the filter circuit417, the gain amplifier circuit 419 and the signal generator circuit 500may be implemented by using any adaptive mixer circuit, filter circuit,gain amplifier circuit and signal generator circuit in the related art,which are not particularly limited by the invention. Therefore, enoughteaching, suggestion, and implementation illustration for internalcircuit structures and implementations of the mixer circuit 415, thefilter circuit 417, the gain amplifier circuit 419 and the signalgenerator circuit 500 may be obtained with reference to common knowledgein the related art, which are not repeated hereinafter.

FIG. 10 to FIG. 11 respectively illustrate schematic diagrams of signaldemodulation according to a plurality of embodiments of the invention.Referring to FIG. 10 to FIG. 11, the amplified touch sensing signal Vsmay be a periodical signal Vs10 or Vs11 having a period T in theexemplary embodiments. In the present embodiment, by using thedemodulating signal Vde1, the demodulating circuit 414 demodulates theamplified touch sensing signal Vs10 or Vs11 to extract a carrier signalthat includes the information related to the gesture 200 from theamplified touch sensing signal Vs10 or Vs11 during a part time intervalt1 or t2 of the period T. Taking the signal demodulation illustrated inFIG. 10 for example, the part time interval t1 that the carrier signalis extracted substantially locates in a middle region of the high level,and thus the issue of time jitter is reduced in the present embodiment.Demodulating the amplified touch sensing signal Vs10 or Vs11 by usingother demodulating signals Vde0 and Vde2 to Vde4 may be deduced byanalogy, and it is not further described herein.

FIG. 12 illustrates a schematic circuit diagram of a touch sensingchannel according to another embodiment of the invention. Referring toFIG. 12, the touch sensing channel 510 of the present embodiment issimilar to the touch sensing channel 410 illustrated in FIG. 9, and themain difference therebetween, for example, lies in that the demodulatingcircuit 514 further includes an analog-to-digital converter circuit 516.The analog-to-digital converter circuit 516 is located before the mixercircuit 515, and connected between the operational amplifier 511 and themixer circuit 515. The analog-to-digital converter circuit 516 convertsthe touch sensing signal Vs of an analog format to the touch sensingsignal Vs of a digital format. In another embodiment of the invention,the analog-to-digital converter circuit may also be located after themixer circuit.

FIG. 13 illustrates a schematic circuit diagram of a touch sensingchannel according to another embodiment of the invention. Referring toFIG. 13, the touch sensing channel 610 of the present embodiment issimilar to the touch sensing channel 410 illustrated in FIG. 9, and themain difference therebetween, for example, lies in that the demodulatingcircuit 614 further includes an analog-to-digital converter circuit 616.The analog-to-digital converter circuit 616 is located after the mixercircuit 615, and connected between the mixer circuit 615 and the filtercircuit 617. The analog-to-digital converter circuit 616 converts thedemodulated touch sensing signal Vs' of an analog format to thedemodulated touch sensing signal Vs' of a digital format.

FIG. 14 illustrates a schematic block diagram of a touch sensing channelaccording to another embodiment of the invention. Referring to FIG. 14,the touch sensing channel 710 of the present embodiment is similar tothe touch sensing channel 310 illustrated in FIG. 3, and the maindifference therebetween, for example, lies in that the touch sensingchannel 710 includes a plurality of operational amplifier circuits 712_1to 712_4 and further includes a multiplexer circuit 716.

In the present embodiment, the multiplexer circuit 716 is coupledbetween the operational amplifier circuits 712_1 to 712_4 and thedemodulating circuit 714. The multiplexer circuit 714 sequentiallyselects one of the operational amplifier circuits 712_1 to 712_4, andpass the amplified touch sensing signal Vs1, Vs2, Vs3 or Vs4 to thedemodulating circuit 714 for signal demodulation. In the presentembodiment, the touch sensing channel 710 demodulates the amplifiedtouch sensing signals Vs1 to Vs4 in a manner of time-divisionmultiplexing (TDM). The multiplexer circuit 714 may sequentially orrandomly demodulate the amplified touch sensing signals Vs1 to Vs4 tooutput the processed touch sensing signal.

For example, the multiplexer circuit 714 may select the operationalamplifier circuit 712_1. The operational amplifier circuits 712_2 to712_4 are disabled. The operational amplifier circuit 712_1 transmitsthe amplified touch sensing signal Vs1 to the multiplexer circuit 714,and the multiplexer circuit 714 passes the amplified touch sensingsignal Vs1 to the demodulating circuit 714 for signal demodulation. Inthe meanwhile, the operational amplifier circuits 712_2 to 712_4 may bedisable to stop working, and thus the operational amplifier circuits712_2 to 712_4 do not transmit the amplified touch sensing signals Vs2to Vs4 to the multiplexer circuit 714. Next, the multiplexer circuit 714may select the operational amplifier circuit 712_2. The operationalamplifier circuits 712_1, 712_3 and 712_4 are disabled. The operationalamplifier circuit 712_2 transmits the amplified touch sensing signal Vs2to the multiplexer circuit 714, and the multiplexer circuit 714 passesthe amplified touch sensing signal Vs2 to the demodulating circuit 714for signal demodulation. The operation of the operational amplifiercircuits 712_3 and 712_4 can be deduced by analogy, and it is notfurther described herein.

In the present embodiment, if one of the operational amplifier circuits712_1 to 712_4 is enabled, the other operational amplifier circuits aredisabled. By adopting the manner of time-division multiplexing forsignal demodulation, the filter circuit of the demodulating circuit 714is further simplified. For example, a ROM table thereof is relativelysmall.

In the present embodiment, the operational amplifier circuits 712_1 to712_4, the multiplexer circuit 716, and the demodulating circuit 714 maybe implemented by using any adaptive operational amplifier circuit,multiplexer circuit, and demodulating circuit in the related art, whichare not particularly limited by the invention. In one embodiment, theoperational amplifier circuits 712_1 to 712_4 and the demodulatingcircuit 714 may be implemented by using the operational amplifiercircuit and the demodulating circuit illustrated in FIG. 9, FIG. 12 orFIG. 13. Therefore, enough teaching, suggestion, and implementationillustration for internal circuit structures and implementations of theoperational amplifier circuits 712_1 to 712_4, the multiplexer circuit716, and the demodulating circuit 714 may be obtained with reference tocommon knowledge in the related art, which are not repeated hereinafter.

FIG. 15 is a flowchart illustrating steps of a signal demodulatingmethod in an embodiment of the invention. Referring to FIG. 9 and FIG.15, the signal demodulating method is at least adapted to the touchsensing channel 410 in FIG. 9, but the invention is not limited thereto.The signal demodulating method at least includes the following steps. Instep S500, the touch sensing channel 410 receives the touch sensingsignal 114, and amplifies the touch sensing signal 114 by compensatingthe received the touch sensing signal 114 via the direct-current voltageVDS. In step S510, the touch sensing channel 410 demodulates theamplified touch sensing signal Vs by mixing the amplified touch sensingsignal Vs' with the demodulating signal Vde that the waveform includes aflat region, so as to output the processed touch sensing signal 116.

Besides, the signal demodulating method described in the presentembodiment of the invention is sufficiently taught, suggested, andembodied in the embodiments illustrated in FIG. 1 to FIG. 14, andtherefore no further description is provided herein.

In summary, in the exemplary embodiments of the invention, theoperational amplifier circuit compensates the received touch sensingsignal via a direct-current voltage, and thus a compensation capacitoris unnecessary. The circuit structure of the operational amplifiercircuit is further simplified. The demodulating circuit demodulates theamplified touch sensing signal with a demodulating signal that thewaveform includes a flat region by a low-bit multiplier. Accordingly,the circuit performance is enhanced.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A touch sensing circuit, comprising: a pluralityof touch sensing channels, each comprising: at least one operationalamplifier circuit, configured to receive a touch sensing signal, andamplify the touch sensing signal, wherein the at least one operationalamplifier circuit comprises an operational amplifier having an invertingend, and the inverting end is configured to receive the touch sensingsignal and a direct-current voltage and to compensate the received touchsensing signal by the direct-current voltage; and a demodulatingcircuit, coupled to the at least one operational amplifier circuit, andconfigured to demodulate the amplified touch sensing signal by mixingthe amplified touch sensing signal with a demodulating signal.
 2. Thetouch sensing circuit according to claim 1, wherein the at least oneoperational amplifier circuit comprises a plurality of operationalamplifier circuits, and each of the touch sensing channels comprises: amultiplexer circuit, coupled between the operational amplifier circuitsand the demodulating circuit, and configured to sequentially select oneof the operational amplifier circuits, and pass the amplified touchsensing signal to the demodulating circuit.
 3. The touch sensing circuitaccording to claim 1, wherein the amplified touch sensing signal is aperiodical signal having a period, and the demodulating circuitdemodulates the amplified touch sensing signal to extract a carriersignal from the amplified touch sensing signal during a part timeinterval of the period.
 4. The touch sensing circuit according to claim1, wherein the operational amplifier further has an output end, and theat least one operational amplifier circuit further comprises animpedance device coupled between the inverting end and the output end.5. The touch sensing circuit according to claim 1, wherein thedemodulating circuit comprises: a mixer circuit, coupled to the at leastone operational amplifier circuit, configured to receive and mix theamplified touch sensing signal and the demodulating signal to generatethe demodulated touch sensing signal; a filter circuit, coupled to themixer circuit, configured to receive the demodulated touch sensingsignal, and reduce noise of the demodulated touch sensing signal; and again amplifier circuit, coupled to the filter circuit, configured toreceive and amplify the demodulated touch sensing signal that the noiseis reduced.
 6. The touch sensing circuit according to claim 5, whereinthe demodulating circuit further comprises: an analog-to-digitalconverter circuit, coupled to the mixer circuit and located before themixer circuit or after the mixer circuit, and configured to convert thetouch sensing signal of an analog format to the touch sensing signal ofa digital format.
 7. A touch sensing circuit, comprising: an operationalamplifier circuit, configured to receive a touch sensing signal, andamplify the touch sensing signal, wherein the operational amplifiercircuit comprises an inverting end, and the inverting end is configuredto receive the touch sensing signal and a direct-current voltage and tocompensate the received touch sensing signal by the direct-currentvoltage; and a demodulating circuit, coupled to the operationalamplifier circuit, and configured to demodulate the amplified touchsensing signal by mixing the amplified touch sensing signal with ademodulating signal.
 8. The touch sensing circuit according to claim 7,wherein the amplified touch sensing signal is a periodical signal havinga period, and the demodulating circuit demodulates the amplified touchsensing signal to extract a carrier signal from the amplified touchsensing signal during a part time interval of the period.
 9. The touchsensing circuit according to claim 7, further comprising: an impedancedevice coupled between the inverting end and an output end of theoperational amplifier circuit.
 10. The touch sensing circuit accordingto claim 7, wherein the demodulating circuit comprises: a mixer circuit,coupled to the operational amplifier circuit, configured to receive andmix the amplified touch sensing signal and the demodulating signal togenerate the demodulated touch sensing signal; a filter circuit, coupledto the mixer circuit, configured to receive the demodulated touchsensing signal, and reduce noise of the demodulated touch sensingsignal; and a gain amplifier circuit, coupled to the filter circuit,configured to receive and amplify the demodulated touch sensing signalthat the noise is reduced.
 11. The touch sensing circuit according toclaim 10, wherein the demodulating circuit further comprises: ananalog-to-digital converter circuit, coupled to the mixer circuit andlocated before the mixer circuit or after the mixer circuit, andconfigured to convert the touch sensing signal of an analog format tothe touch sensing signal of a digital format.
 12. A signal demodulatingmethod, adapted to a touch sensing circuit, the signal demodulatingmethod comprising: configuring at least one operational amplifiercircuit, wherein the at least one operational amplifier circuitcomprises an operational amplifier having an inverting end; receiving,at the inverting end, at least one touch sensing signal and adirect-current voltage to amplify the at least one touch sensing signalby compensating the received at least one touch sensing signal by thedirect-current voltage; and demodulating the amplified at least onetouch sensing signal by mixing the amplified at least one touch sensingsignal with a demodulating signal.
 13. The signal demodulating methodaccording to claim 12, wherein the at least one touch sensing signalcomprises a plurality of touch sensing signals, and the signaldemodulating method further comprises: sequentially selecting one of theamplified touch sensing signals to be demodulated.
 14. The signaldemodulating method according to claim 12, wherein the amplified atleast one touch sensing signal is a periodical signal having a period,and in the step of demodulating the amplified at least one touch sensingsignal by mixing the amplified at least one touch sensing signal withthe demodulating signal, the amplified at least one touch sensing signalis demodulated to extract a carrier signal from the amplified at leastone touch sensing signal during a part time interval of the period. 15.A signal demodulating method, adapted to a touch sensing circuitcomprising an operational amplifier, the signal demodulating methodcomprising: receiving, at an inverting end of the operational amplifier,a touch sensing signal and a direct-current voltage; amplifying thetouch sensing signal by compensating the touch sensing signal by thedirect-current voltage; and demodulating the amplified touch sensingsignal by mixing the amplified touch sensing signal with a demodulatingsignal.